Adaptive method and arrangement for implementing incremental redundancy in reception

ABSTRACT

A method and an arrangement are presented for processing received data blocks in a digital radio receiver. Received data blocks are equalized ( 306 ) and channel decoded, ( 309 ) after which they are checked ( 310 ) for errors. Additionally there is monitored ( 303 ) the amount of received but not yet equalized and channel decoded data. As a response to a finding indicating that an equalized and channel decoded data block contains errors ( 310 ), it is checked ( 313 ) whether the amount of received but not yet equalized and channel decoded data is below a certain threshold. As a response to a finding indicating that the amount of received but not yet equalized and channel decoded data is below said threshold, the data block which was found to contain errors is iteratively equalized and channel decoded. By adaptively allowing iterative equalization and channel decoding, retransmissions may be avoided.

TECHNOLOGICAL FIELD

[0001] The invention relates in general to the technology of enhancingthe possibilities of a digital wireless receiver to correctlyreconstruct the information content of a received block of data.Especially the invention relates to the technology of only adaptivelyproviding incremental redundancy to the received block of data to enablethe reconstruction of its information content.

BACKGROUND OF THE INVENTION

[0002] Many digital radio systems employ a sophisticated arrangement ofacknowledgements and retransmissions to enable the receiver to correctlyreconstruct the information content of each received data block. This isespecially the case in packet switched radio connections used to conveynon-real time services, because the nonpredictable need forretransmissions is ill suited for the tight timing requirements of realtime services. A retransmission means that in some way or another thereceiver makes the transmitter aware of that a block of data was notreceived in a good enough shape, so the transmitter must transmit atleast a part thereof again.

[0003] Retransmitting the same piece of data several times consumestransmission time and bandwidth, which are scarce in multiple usersystems like cellular radio networks. Incremental redundancy in generalmeans that the transmitter tries to find and transmit the smallestpossible portion of the previously transmitted information that would beenough for the receiver to perform its task of reconstructing theinformation content. Many known incremental redundancy schemes rely onthe fact that the first version of the transmitted data block ispunctured, i.e. a predefined pattern of bits thereof are intentionallyomitted at the transmission stage. If the conditions of reception aregood, the receiver is able to use the remaining parts of the data blockto bridge the punctured gaps. If the first decoding round isunsuccessful, the receiver asks the transmitter to additionally give anumber of the originally punctured bits.

[0004] Also incremental redundancy arrangements have their drawbacks.Even if the limited retransmissions are much smaller in bit number thancompletely retransmitted blocks, they still consume radio resources. Theconcept of retransmissions, be they complete or partial, inherentlyintroduces delay because the receiver must react to an unsuccessfuldecoding attempt and find a scheduled turn for asking for theretransmission, and the transmitter must receive and decode the requestand react thereto as well as wait for the next suitable time instant fortransmitting the requested additional bits.

SUMMARY OF THE INVENTION

[0005] It is an object of the present invention to provide a method andan arrangement for efficiently using the transmission and receptionresources in a radio system where retransmission is possible.

[0006] The objects of the invention are achieved by equipping thereceiver with a reception buffer, an iterative decoder and a decisionelement which is arranged to consider the filling ratio of the receptionbuffer in deciding, whether to iterate on an already received amount ofdata or to ask for a retransmission.

[0007] The method according to the invention is characterized in that itcomprises the steps of:

[0008] monitoring the amount of received but not yet equalized andchannel decoded data,

[0009] as a response to a finding indicating that an equalized andchannel decoded data block contains errors, checking whether the amountof received but not yet equalized and channel decoded data is below acertain threshold and

[0010] as a response to a finding indicating that the amount of receivedbut not yet equalized and channel decoded data is below said threshold,iteratively equalizing and channel decoding the data block which wasfound to contain errors.

[0011] The invention applies also to a radio receiver comprising, in areceiver chain, a series coupling of an equalizer, a channel decoder andan error detector; it is characterized in that it comprises a decisionelement coupled to said error detector, which decision element isarranged to

[0012] monitor the amount of received but not yet equalized and channeldecoded data,

[0013] as a response to the error detector indicating that an equalizedand channel decoded data block contains errors, check whether the amountof received but not yet equalized and channel decoded data is below acertain threshold and

[0014] as a response to a finding indicating that the amount of receivedbut not yet equalized and channel decoded data is below said threshold,initiate iterative equalization and channel decoding of the data blockwhich was found to contain errors.

[0015] The concept of iterative equalization and decoding is known assuch from e.g. A. Picart, P. Didier and A. Glavieux: “Turbo-Detection: Anew approach to combat channel frequency selectivity”, ICC 97, or G.Bauch and V. Franz: “Iterative Equalization and Decoding for the GSMSystem” in Vehicular Technology Conference (VTC), IEEE, May 1998; thesepublications are incorporated herein by reference. Iterativeequalization and decoding means that decoding decisions from a certaindecoding round are fed back as a kind of a priori information into a newsignal processing round where the same block of digital data isequalized and decoded anew.

[0016] According to the invention, received digital data is temporarilystored in a reception buffer. A block of data obtained from the bufferis equalized and decoded; a deinterleaving step is also needed ifinterleaving was used in the transmitting end to distribute originallyadjacent bits according to some interleaving scheme. After decoding thereceiver calculates a checksum or uses some other feature of the decodeddata to examine, whether or not it contains errors. If errors are found,a decision element in the receiver has the possibility of obtainingdecoding decisions from the decoder and feeding them back to theequalizer for an iterative equalization and decoding round. In makingthe decision the decision element considers the filling ratio of thereception buffer to determine, whether enough time remains for theiterative equalization and decoding round to be performed. If thereception buffer appears to fill over a certain limit, the decisionelement disables further iterative decoding of the same data block andasks for a retransmission instead or declares the data block invalid ifretransmissions are not possible.

BRIEF DESCRIPTION OF DRAWINGS

[0017] The novel features which are considered as characteristic of theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings.

[0018]FIG. 1 illustrates a signal processing principle according to anembodiment of the invention,

[0019]FIG. 2 illustrates a radio transceiver according to an embodimentof the invention and

[0020]FIG. 3 illustrates a method according to an embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0021]FIG. 1 illustrates a signal processing principle where a receivedsignal is directed into a buffer memory 101 for temporary storage. Anindependently decodable piece of the stored signal, designated here as ablock, is taken into an equalizer and channel decoder unit 102 which iscapable of iterative equalization and decoding. A first pass throughunit 102 does not involve iteration, in the hope that already the firstequalization and decoding attempt will result in an error-free block ofdecoded digital information. In order to check for the occurrence oferrors, the output of the equalizer and channel decoder unit 102 istaken into an error detecting unit 103. The latter may be e.g. a knownCRC (Cyclic Redundancy Check) checksum calculator which calculates achecksum and compares it against a corresponding checksum value includedwithin the block of received data. If the block of decoded digitalinformation is found to contain no errors, it can be passed on to theapplication to the use of which it was intended.

[0022] However, there is a monitoring unit 104 which is coupled both tothe buffer memory 101 to keep up with the filling ratio thereof and tothe error detecting unit 103 to find out if the decoded block of digitaldata was found to contain errors. If the monitoring unit 104 learns fromthe error detecting unit that the decoded block of digital datacontained errors, it checks the current filling situation of the buffermemory 101. If the buffer memory 101 is empty or nearly empty, there isenough time for at least one iteration round in unit 102 before a newblock of received data has to be read in from the buffer memory 101. Insuch a case the monitoring unit instructs the equalizer and channeldecoder unit 102 to take the previously obtained decoding decisions (ora subset thereof, according to some suitable principle of iterativeequalization and decoding) as “a priori” information to the input of theequalizer stage and to perform a new round of equalization and channeldecoding.

[0023] In a situation where errors are detected by unit 103 it may alsohappen that the buffer memory 101 is found to be filled up to apredetermined limit. In such a case an attempt to iteratively equalizeand decode a previously received block of data would cause anunacceptable delay of reading the next block from the buffer memory 101,so further iterations on the previously received block must be disabled.In order to facilitate the correction of errors the monitoring unit 104may initiate a retransmission request concerning the block which thereceiver was unable to equalize and decode correctly.

[0024]FIG. 2 illustrates a radio transceiver according to an embodimentof the invention. An antenna 201 is coupled to a duplexer 202 forseparating received signals from signals to be transmitted. Thereception branch output of the duplexer 202 is coupled to a receiverblock 203 which comprises, in a way known as such, components likeamplifiers, filters and A/D converters. The output of the receiver block203 consists of digital samples that represent a received signal in anunequalized and undecoded form.

[0025] The output of the receiver block 203 is coupled to the input of aFIFO-type (First In-First Out) reception buffer 204 which is capable ofstoring a certain number of samples at a time. We will analyze theoptimal dimensioning of the buffer 204 in more detail later. The buffer204 comprises also a control output from which the filling ratio of thebuffer can be read. The data output of the buffer 204 is coupled to anequalizer 205, from which the path of received data continues through adeinterleaver 206, a decoder 207 and error detection unit 208 to a datasink 209. From the decoder 207 there is also an output to areinterleaver 210 which in turn is coupled to an additional input in theequalizer 205. Together the loop consisting of the equalizer, thedeinterleaver 206, the decoder 207 and the feedback connection from thelatter through the reinterleaver 210 to the equalizer 205 constitutes aso-called turbo equalizer 205′.

[0026] There are control connections from the error detection unit 208and the reception buffer 204 to a decision element 211 from which thereare control outputs to the reinterleaver 210 and a retransmissionrequest generator 212. In the transmitter side of the transceiver inFIG. 2 there is a data source 213 which, together with theretransmission request generator 212, is coupled through a transmissionmultiplexer 214 to a transmitter block 215 which includes, in a mannerknown as such, the necessary means for transforming a digital bitstreaminto modulated radio frequency oscillations. The output of thetransmitter block 215 is coupled to the transmission branch input of theduplexer 202.

[0027]FIG. 3 illustrates an exemplary method of operation for the radiotransceiver of FIG. 2. The reception of radio signals and theirconversion into stored digital samples takes place in the loopconsisting of steps 301 and 302. The time schedule of circulating thisreception loop is dictated by the transmission schedule employed in theradio system in question, so conceptually the reception loop may beregarded almost as separated from the signal processing operations thataim at equalizing and decoding the received signals. The RX buffermemory 204 in FIG. 2 serves as the means for exchanging informationbetween the reception loop and the equalization and channel decodingprocesses. The step of monitoring the filling ratio of the buffer memoryis shown as step 303 in FIG. 3.

[0028] At step 304 a group of samples are read from the RX buffermemory. In the most often encountered case the samples are those thatrepresent a new block of data to be equalized and channel decoded.However, at step 305 it is checked whether that is the case or whetherthe samples are complementary samples to some previously processed blockthe equalization and channel decoding of which was found to beimpossible without a retransmission. The samples of a newly read blockare passed on to an equalization step 306, whereas complementary samplesto some previous block are combined with the previous samples of thatblock in step 307. The invention does not limit the selection ofcombination strategy: indeed in some cases it may be better to ask for acomplete retransmission of a seriously defective data block in whichcase step 307 means that the whole previous sample group representingthe defective block is replaced with fresh new samples.

[0029] An equalization, deinterleaving and decoding round consists ofsteps 306, 308 and 309. At step 310 the channel decoded block from step309 is checked for errors. In the optimal case no errors are found, sothe block may be output at step 311, after which the receiver returns tostep 304 (note that the loop consisting of steps 301 and 302 has beenindependently running all the time). If errors are found at step 310,there follows at step 312 a check for the number of already performediteration rounds on the same data. Iterative equalization and decodingprocedures have a tendency to converge towards a certain final resultafter a relatively small number of iteration rounds, so it may beadvantageous to set a limit for the allowed number of iteration rounds.

[0030] The requirement of no errors can be somewhat generalized bystating that the detected erroneousness of the equalized and channeldecoded data block must be below a certain threshold.

[0031] Taken that the number of already performed iteration rounds isfound to be within limits at step 312, the receiver checks at steps 313the current filling ratio of the RX buffer. Even if the buffer is notfilled up to the threshold of disabling further iterations, it may beadvantageous to check for other existing timing constraints; this isdone at step 314. Only after a positive decision at step 314 thereceiver enables the reinterleaving of the most recently obtaineddecoding decisions at step 315. The results are then fed as a prioriinformation to a further equalization and decoding round starting atstep 306.

[0032] If it is found at step 312 that the number of allowed iterationrounds has been reached, or if the reception buffer is found to be fullenough at step 313, or if some other timing constraint is found to applyat step 314, there follows a transition to step 316 where the receiverchecks, whether retransmissions are allowable. The allowability ofretransmissions is usually a characteristic of the radio bearer which isused to convey the blocks of digital data. If retransmissions areallowed, a retransmission request is initiated at step 317. Otherwisethe block which was found to be erroneous is declared as invalid at step318. In any case the receiver returns to step 304 in order to continuethe processing of received information.

[0033] The selection of decision criteria shown as steps 312, 313 and314 may be simplified from that shown in FIG. 3. For example, thereceiver may rely completely on monitoring the filling ratio of thereception buffer and omit any additional checks of allowed number ofiterations or remaining time. Alternatively the check of remaining timemay be used as the single decision criterion, especially if thetransmission rate is known to be constant so that the time elapsed sincethe moment of reading a certain block from the reception buffer is knownto unequivocally correspond to the filling ratio of the receptionbuffer. Even the maximum number of iterations may serve as the solecriterion for decisions, if it can be guaranteed that the receiver isalways able to perform the maximum number of iterations in less than apredetermined maximum time.

[0034] However, also other decision criteria can be applied: for exampleif the error detection arrangement allows detecting the number oferrors, the receiver may check, whether a certain iteration roundproduced any reduction in that number. If the number of errors stays thesame despite of consecutive iterations, it is not worth the effort toiterate any more even if timing or other constraints would allowadditional iteration rounds.

[0035] The form in which the decoding decisions are fed back throughreinterleaving to the equalization process is worth some consideration.At the error detection stage the original block of multiple-valueddigital samples must have been converted to so-called hard decisions,meaning that the value of each bit may be only exactly 0 or exactly 1.However, up to the very last stages of channel decoding the principle ofsoft decisions may be applied, where each bit value is represented by amere probability of it being either 0 or 1. The invention does notdictate whether soft or hard decisions are fed back into the iterativeequalization and channel decoding process, although in many cases betterresults are obtained by feeding back the soft decisions.

[0036] In the foregoing we have not given a precise definition of whatshould be regarded as a critical threshold for the filling ratio of thereceiving buffer. The invention does not call for a specific definition,because the criticality of the filling ratio depends both on theprocessing capability of the equalization and channel decoding loop aswell as on the maximum allowable overall delay in processing receivedpackets: if the equalization and channel decoding loop is very fast, arelatively large portion of the following data block may be allowed toaccumulate into the buffer, and if delay is not a problem, even severaldata blocks may be waiting in the buffer while the equalization andchannel decoding loop is trying to reconstruct a particularly badlycorrupted data block. In practical communication situations, especiallywithin the framework of the so-called third generation digital cellularnetworks, it may well be that every radio bearer has its ownindividually defined delay limits, so it is advantageous to make thecritical threshold for the filling ratio of the receiving bufferdynamically changeable. To give an estimate, with the technology of thepriority date of the present patent application it is believed that ifno more than one fourth of a new data block is allowed to accumulateinto the receiving buffer before disabling further iteration on theprevious data block, the method according to the invention would notcause additional delay to the continuous reception and processing ofdata blocks.

[0037] A multitude of known methods exist for monitoring the fillingratio of a buffer memory. The invention does not limit the selection ofsuch method.

[0038] The features of the invention that are presented in the dependingpatent claims are freely combinable unless otherwise explicitly stated.

1. A method for processing received data blocks in a digital radioreceiver, comprising the steps of: equalizing and channel decoding areceived data block, checking the equalized and channel decoded datablock for errors, monitoring the amount of received but not yetequalized and channel decoded data, as a response to a findingindicating that an equalized and channel decoded data block containserrors, checking whether the amount of received but not yet equalizedand channel decoded data is below a certain threshold and as a responseto a finding indicating that the amount of received but not yetequalized and channel decoded data is below said threshold, iterativelyequalizing and channel decoding the data block which was found tocontain errors.
 2. A method according to claim 1 , comprising the stepof temporarily storing data belonging to received data blocks in abuffer memory prior to their equalization and channel coding, so thatthe step of monitoring the amount of received but not yet equalized andchannel decoded data comprises the substep of monitoring the fillingratio of said buffer memory.
 3. A method according to claim 1 , whereinthe step of monitoring the amount of received but not yet equalized andchannel decoded data comprises the substep of measuring the time passedsince the beginning of the equalization and channel decoding of acurrently equalized and channel decoded data block.
 4. A methodaccording to claim 1 , wherein the step of iteratively equalizing andchannel decoding the data block which was found to contain errorscomprises the substeps of: after each iterative round of equalizing andchannel decoding, checking the iteratively equalized and channel decodeddata block for errors and as a response to a finding indicating that thedetected erroneousness of the iteratively equalized and channel decodeddata block is below a certain threshold, disabling further iterationrounds on the same data block.
 5. A method according to claim 4 ,wherein the step of iteratively equalizing and channel decoding the datablock which was found to contain errors additionally comprises thesubsteps of: calculating the number of iterative rounds of equalizingand channel decoding the data block and as a response to the number ofiterative rounds of equalizing and channel decoding reaching a certainlimit, disabling further iteration rounds on the same data block.
 6. Amethod according to claim 4 , wherein the step of iteratively equalizingand channel decoding the data block which was found to contain errorsadditionally comprises the substeps of: after each iterative round ofequalizing and channel decoding, checking whether a time constraintallows further iteration and as a response to a finding indicating thatsaid time constraint does not allow further iteration, disabling furtheriteration rounds on the same data block.
 7. A method according to claim4 , comprising the steps of: examining, whether retransmissions areallowed for such data blocks for which the detected erroneousness is notbelow said threshold, and as a response to a finding indicating thatretransmissions are allowed, initiating a request for retransmissionconcerning a data block for which the detected erroneousness was foundnot to be below said threshold.
 8. A method according to claim 1 ,comprising the step of deinterleaving a block of data betweenequalization and channel decoding, so that the step of iterativelyequalizing and channel decoding the data block which was found tocontain errors comprises the substep of reinterleaving those parts ofthe data block which are fed back to equalization as a part of theiteration.
 9. A radio receiver comprising: in a receiver chain, a seriescoupling of an equalizer, a channel decoder and an error detector, and adecision element coupled to said error detector; wherein said decisionelement is arranged to monitor the amount of received but not yetequalized and channel decoded data, as a response to the error detectorindicating that an equalized and channel decoded data block containserrors, check whether the amount of received but not yet equalized andchannel decoded data is below a certain threshold and as a response to afinding indicating that the amount of received but not yet equalized andchannel decoded data is below said threshold, initiate iterativeequalization and channel decoding of the data block which was found tocontain errors.
 10. A radio receiver according to claim 9 , comprising,prior to the equalizer in the propagation direction of received signals,a buffer memory, so that said decision element is arranged to monitorthe filling ratio of said buffer memory.
 11. A radio receiver accordingto claim 9 , additionally comprising: a deinterleaver coupled betweenthe equalizer and the channel decoder for deinterleaving equalized butnot yet channel decoded data, and a reinterleaver coupled between thechannel decoder and the equalizer for reinterleaving channel decodeddata which is to be fed as a priori information to an iterative round ofequalization and channel decoding.